1. Field of the Invention
The present invention relates to a method for transmitting data from an RLC layer in a radio communication system, and more particularly, to a method for carrying out a SDU discard function in a radio communication system having an RLC layer.
2. Background of the Related Art
FIG. 1 illustrates an architectural diagram of a general radio communication system, having an architecture of a radio interface protocol according to a radio access network standard of an European IMT-2000 UMTS (universal mobile telecommunication system).
Referring to FIG. 1, a radio link control (RLC) layer in a radio communication system as a second layer of 3GPP is a protocol layer controlling a data link. Moreover, the RLC layer corresponds to a second layer of an OSI 7-layered model. All entities of the RLC layer are embodied by a radio resource control (RRC) layer as an upper layer. The radio interface protocol between a user equipment and a UTRAN (UMTS terrestrial radio access network) includes a physical layer, a data link layer, and a network layer horizontally. Vertically, the protocol architecture is divided into a control plane for transferring a control signal (signaling), and a user plane, for transmitting data information.
Specifically, as shown in FIG. 1 a RRC layer exists on the control plane as a third layer, a RLC layer and a medium access control (MAC) layer exist as second layers, and a physical layer exists as a first layer. Moreover, in the user plane, RLC and MAC layers exist on the second layer, and the physical layer exists on the first layer.
The physical layer provides the upper layer with an information transfer service using various radio transmission techniques and is connected to the MAC layer on the upper layer via transport channels. Data is transported between the MAC and physical layers through the transport channels. The transport channels are divided into a dedicated transport channel, used exclusively for the user equipment, and a common transport channel, used for the user equipments in common.
The MAC layer provides a re-allotment service of a radio resource and MAC parameters. Such a service demands the re-allotment of the radio resource or the change of the MAC parameters. The MAC layer is connected to the RLC layer through a logical channel and provides various logical channels in accordance with the species of the transmission information. Generally, the control channel is used when information of the control plane is transmitted, while a traffic channel is used for transmitting information of the user plane.
The RLC layer provides the establishment and release of a radio link and performs segmentation and concatenation relating to a RLC service data unit (SDU) coming down from an upper layer of the user plane. The size of the RLC SDU is adjusted to fit a processing capacity in the RLC layer. Header information is then added to the SDU to form a protocol data unit (PDU). The RLC SUD is transferred to the MAC layer. In this case, an RLC buffer for storing RLC SDUs or RLC PDUs coming down from the upper layer exists in the RLC layer.
The RRC layer provides an information broadcast service for broadcasting information to all user equipments located at random areas. Moreover, the RRC layer takes charge of a control plane signal processing for a control signal exchange in the third layer to establish, maintain, and release a radio resource between the user equipment and UTRAN. Specifically, the RRC layer also has functions of establishing, maintaining, and releasing a radio access bearer channel as well as allotment, rearrangement, and release of a radio resource necessary for a radio resource access. In this case, the radio access bearer means a service provided by the second layer for data transfer between the user equipment and UTRAN. Namely, ‘establishing one radio access bearer’ means that characteristics of a protocol layer and a channel required for providing a specific service are stipulated and that the respective specific parameters and operational methods are established.
The RLC layer will next be described in greater detail. In accordance with the functions carried out by the RLC layer, there exist three modes. These are the transparent mode, the unacknowledged mode, and the acknowledged mode.
When the RLC operates in the transparent mode, no header information is added to the RLC SDU coming down from the upper layer. Generally, segmentation and reassembly are not performed in the transparent mode. Yet, if necessary, when the radio access bearer is established (configured), it is determined whether the segmentation and reassembly functions are used or not.
When the RLC operates in the unacknowledged mode, retransmission is not backed up even if the transmission of RLC PDU fails. Therefore, a receiving side does not demand the retransmission when data is lost or when problems occur during transmission. Instead, the receiving side discards the related data. Services using the unacknowledged mode include a cell broadcast service, a voice service using an IP network (voice over IP service), and the like.
When the RLC operates in the acknowledged mode, the retransmission is backed up when the transmission of a packet fails. Thus, the RLC layer of a transmitting side receives status information from the receiving side used to determine the success of the transmission. The transmitting side RLC layer then retransmits the RLC PDUs demanded for retransmission. In the acknowledged mode, the RLC SDU received by the RLC layer from the upper layer is, if necessary, divided into pre-defined sizes by segmentation/concatenation. The RLC SDU then becomes RLC PDUs, to which header information including sequence numbers are added. The RLC PDUs are stored in a RLC buffer according to the sequence numbers.
The stored RLC PDUs amounting to the exact number demanded by the MAC layer are transferred to the MAC layer, where the transmission is carried out in accordance with the order of the sequence numbers. From the RLC layer of the transmitting side, a firstly-transmitted RLC PDU is transmitted according to the order of the sequence numbers. Therefore, the RLC layer of the receiving side checks the received sequence numbers so as to demand retransmission of transmission-failed data from the transmitting side RLC layer.
When the RLC SDUs (service data units) having come down to the RLC layer in the transmitting side radio communication system fail to be transmitted to the receiving side radio communication system, the transmitting side needs to discard the SDUs in order to prevent a transmission buffer from becoming overloaded.
A function of discarding the SDUs (service data units) is used by the RLC layer, which is embodied in the RRC layer as well.
There are two methods for carrying out the SDU discard function. The first method is a time-based SDU discard method. The time-based SDU discard method uses a timer to carry out the discarding of SDUs. The second SDU discard method depends on limiting the number of transmissions. Thus the first method measures the time during which a SDU stays, while the second method measures how many times a PDU is retransmitted. In this case, the target to be discarded is SDU.
Regarding the time-based SDU discard method, when RLC SDUs come down to the RLC layer from the upper layer of the transmitting side, a discard-timer is driven. The discard-timer counts the time during which each RLC SDU stays in the RLC layer.
Subsequently, if the corresponding RLC SDU fails to be transmitted to the receiving side before the discard-timer expires, or if an ACK (acknowledgment) signal fails to be received from the receiving side even if the corresponding RLC SDU is transmitted to the receiving side, then the receiving side radio communication system discards all of the RLC PDUs including the corresponding SDU.
On the other hand, if an RLC SDU, which is not to be discarded, as well as the RLC SDU to be discarded are both included in the same RLC PDU, the transmitting side would not discard the corresponding RLC PDU.
The second SDU discard method functions by limiting the number of transmissions. In the second method, the RLC layer of the transmitting side counts the number of transmissions of the respective RLC PDUs toward the receiving side. Specifically, a counter VT-DAT, which counts the transmission number of the respective RLC PDUs, is initiated to operate. The counter VT-DAT increases the count value by 1 every time one RLC PDU is transmitted to the receiving side.
The transmission-possible number of a specific RLC PDU is defined as a maximum variable MaxDAT. If the count value of the counter VT-DAT becomes equal to or higher than the maximum variable MaxDAT, SDUs included in the corresponding RLC PDU and all the RLC SDUs having been transmitted before the corresponding RLC PDU are discarded.
FIG. 2A illustrates a related art method of transmitting discard information of RLC SDUs discarded using the time-based RLC SDU discard method to the receiving side.
Referring to FIG. 2A, the RLC layer receives RLC SDU0, RLC SDU1, RLC SDU2, and RLC SDU3 from the upper layer, and transforms them into RLC PDU forms. The RLC PDU forms are successively transmitted to the receiving side. In this case, the RLC SDUs are data units stipulated by the upper layer.
The respective SDUs are loaded on at least one RLC PDU so as to be transmitted to the receiving side. SDU0, SDU1, SDU2, and SDU3 include PDU0/PDU1, PDU1, PDU1/PDU2/PDU3, and PDU3 to PDU7, respectively. The sequence numbers, SN 0 to 7, are given to the PDUs PDU0 to PDU7 in order, respectively. Thus, the sequence-numbered PDU is transmitted toward the receiving side.
After the previously-set time during which SDU0 to SDU3 may stay in the transmitting side expires, PDU0 and PDU1 are transmitted to the receiving side. PDU2 and PDU3, however, are lost during the transmission. Meanwhile, PDU4 to PDU 7 are not yet transmitted to the receiving side from the transmitting side.
As mentioned above, once the previously-set time for SDU0 to SDU3 expires, the transmitting side discards PDU0 to PDU7 corresponding to SDU0 to SDU3 in the internal transmission buffer so as to transfer the corresponding discard information, which is loaded on MRW SUFI (move receiving window super field), to the receiving side.
FIG. 2B illustrates formats and parameters of the MRW SUFI which are transmitted in the scenario illustrated in FIG. 2A.
Referring to FIG. 2B, a parameter LENGTH is constructed with 4 bits and indicates the number of discarded SDUs. Other parameters SN_MRW1 to SN_MRW4 indicate sequence numbers of the PDUs respectively. Each of the parameters SN_MRWi (i=1, 2, 3, and 4) represents the sequence number of the corresponding PDU including the end of each of the discarded SDUs. And, each of the parameters SN_MRWi (i=1, 2, 3, and 4) is constructed with 12 bits.
The 12 bit SN_MRWi indicates a sequence number SN of the first PDU in a PDU or PDUs having the data belonging to the SDU next to the ith discarded SDU. That is, considering every SDU unit, the SN_MRWi (i is a positive integer) is the information commanding that the first PDU having the data of the SDU is to be transmitted right after the discarded SDU.
As shown in FIG. 2B, the parameter LENGTH of the MRW SUFI indicating the number of the discarded SDUs is represented by “0100”. Meanwhile, SN_MRW1 is represented by “1,” which is a sequence number of PDU1, since an end of SDU0 belongs to PDU1. The SN_MRW2 is represented by “1,” which is the sequence number of PDU1 since an end of SDU1 belongs to PDU1. The SN_MRW3 is represented by “3,” which is a sequence number of PDU3 since an end of SDU2 belongs to PDU3. Finally, the SN_MRW4 is represented by “7” which is a sequence number of PDU7 since an end of SDU3 belongs to PDU7.
A 4 bit NLENGTH field indicates the data, which defines which SDUs are to be discarded, in the PDU having a sequence number of SN_MRWLENGTH. This indicates which SDUs are discarded when a plurality of SDUs are unable to enter one PDU. For instance, when a value of NLENGTH is set to “0”, the PDU having the sequence number of SN_MRWLENGTH shows that there is no data corresponding to the SDU to be discarded. When a value of NLENGTH is set to “2”, the data corresponding the second SDU from the front is discarded and the data in the PDU having the sequence number of SN_MRWLENGTH is transmitted/received. In FIG. 2A, NLENGTH is represented by “0001”.
The receiving side, which receives the MRW SUFI shown in FIG. 2B as discard information from the transmitting side, discards the PDUs PDU0 to PDU7 corresponding to SDU0 to SDU3 from a receiver buffer inside and moves the receiving window. Subsequently, the receiving side transfers an acknowledged signal to the radio communication system corresponding to the transmitting side. The acknowledged signal commands that MRW_ACK data from the PDU having a sequence number SN of “18” be transmitted to the transmitting side.
The transmitting side, having received the signal from the receiving side, confirms that PDU0 to PDU7 corresponding to the SDU to be discarded have been successfully discarded by the receiving side as well. The transmitting side then starts to transmit the PDU corresponding to the sequence number SN of “8” and so on to the receiving side.
The related art method has many problems and disadvantages. For example, when the timer expires or the previously-set number by the counter is completed, and all of the SDUs related to the transmission are to be discarded, all of the information concerning the discarded SDUs is transmitted to the receiving side. As mentioned above, the sequence number of the corresponding PDU, which indicates the end point of each of the discarded SDUs, comprises 12 bits. The transmission efficiency of the transmitting side is thus greatly reduced if the discarded SDUs are excessive.
Specifically, the transmission of the discard information is designed to make a next receiving window coincide with a next transmission window. Yet, all of the information about the entire discarded SDUs is unnecessarily transmitted to the receiving side. It is enough to inform the receiving side only about the start and end points of a series of the discarded SDUs, of which start and end points are known and which are not transmitted yet, remaining in the internal transmission buffer. Because the excess information is transmitted, a radio resource is wasted.
Moreover, if it is assumed that PDU3 in FIG. 2A has not been transmitted, when the time for which SDU0 to SDU3 are to stay in the transmission buffer expires according to the related art timer-based discard method, the RLC layer of the transmitting side discards SDU0 to SDU3 and transmits such information to the RLC layer. Yet, the information about SDU3, for which transmission has never been attempted, has a zero probability of being received, but is still transmitted to the receiving side. Consequently, another radio resource is wasted.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.